How I finally solved the sliding window AC problem — steps that actually worked

I tried doing the wood frame thing last year with my Frigidaire 8,000 BTU unit, but I actually swapped the wood for 1-inch rigid foam insulation board. It’s a lot easier to cut with a utility knife to get that perfect seal around the sliding glass. I found that the Ivory Garden bracket held up well, but I had to add a small shim because my sill had a weird lip.

For the screen, I went with the magnetic strips you can buy in rolls. I stuck one side to the window frame and the other to the mesh. This makes it way faster to pull the whole thing off when I need to clean the AC filter or if a storm is coming. It gives me about 28 inches of open space next to the unit, which is plenty for the breeze to come through when the humidity finally drops at 2 AM.

One thing I noticed is that the weatherstripping tape can get pretty gummy after a few weeks of the sun hitting it. I started using a layer of painters tape under the heavy-duty stuff so it doesn't ruin the finish on my vinyl windows when I take everthing down in October. It's a bit of a project to set up, but not having to lift that 50-pound unit every day saves my back.

While the rigid foam insulation board undoubtedly provides superior thermal resistance compared to traditional plywood, I find that one must be exceedingly cautious regarding the accumulation of condensation along the interior interface where the cold air from the evaporator coil inevitably makes contact with the non-permeable surface of the foam. Furthermore, for those utilizing the horizontal sliding configuration, I highly suggest incorporating a secondary security bar or a precisely measured PVC pipe segment within the lower track to physically prevent the window from being manipulated from the exterior, as the standard latch mechanism is rendered entirely useless once the glass is positioned to accomodate the cooling unit's chassis.

That setup sounds okay for airflow, but have you ruled out the security risk of leaving a sliding window that accessible at night? Most of those no-screw brackets are fine for weight, but if the glass is slid open, there's often nothing stopping someone from just lifting the whole AC unit or sliding the glass further from the outside.

Also, I'm a bit worried about the vibration. Does that custom mesh realy stay quiet when the wind picks up, or does it rattle? I tried a similar mesh screen once and the constant buzzing against the metal casing at 3 AM drove me crazy.

Quick update—I tried the rigid foam board suggestion and it’s a game changer for getting a tight seal. Much easier than my old wood frame! However, I’m still stuck on the security concern someone brought up. Since the window slides open so easily now, has anyone found a reliable way to lock the track so the unit can't just be lifted out?